Fluorine-containing rubbers have excellent properties such as heat resistance, oil resistance, solvent resistance and chemical resistance as compared with rubbers for general uses, and are widely used as industrial materials in the field where these properties are required.
The fluorine-containing rubbers have been usually used by vulcanizing and molding a vulcanizable fluorine-containing rubber composition, which is prepared by blending a vulcanizing agent, a vulcanization accelerator, an acid receiving agent, a vulcanizing auxiliary, an inorganic filler and the like.
As the above-mentioned mixing components, examples of the vulcanizing agent are a polyamine or its salt, an organic peroxide, a polyhydroxyl compound and the like. As the vulcanization accelerator, examples are an organic tertiary or quaternary compound containing nitrogen or phosphorus and the like, and as the acid receiving agent, examples are a bivalent metal oxide or a hydroxide and the like. As the vulcanizing auxiliary, examples can be mentioned a compound having a plurality of vinyl groups or aryl groups. In addition thereto, there are admixed inorganic fillers, if necessary, such as carbon black, silica, clay, diatom earth, talc, calcium carbonate and the like for the purposes to improve and enhance mechanical properties of mainly the fluorine-containing rubbers.
Further, in order to enhance abrasion resistance of the vulcanized rubber, there are admixed, as a solid lubricant, molybdenum disulfide and graphite or a low molecular weight polytetrafluoroethylene and/or a carbon fluoride as disclosed in JP-B-40168/1981.
Also in order to obtain a high level chemical resistance and elusion resistance which are required in a semiconductor production process and the like, there is a case where a carbon fluoride is admixed as disclosed in JP-A-169845/1987.
These fluorine-containing rubbers are electrically insulative materials having a volume specific resistance exceeding 10.sup.12 .OMEGA.cm. Therefore there was a problem that the molded fluorine-containing rubbers are apt to charge with static electricity thereon, and are susceptible to contamination due to dusts. Thus for the use in the semiconductor production process, it was very difficult to handle those rubbers because of dust generation. Also in a pressure roller, paper feed roller and the like, those rubbers cause troubles that a paper sticks to the roller due to static electricity and toner powders stick or scatter. Also in the use for fuel tubes of automobiles, a spark generated by static electricity becomes a cause for a fire and it is very dangerous.
Therefore it is attempted to admix an electrically conductive material in a fluorine-containing rubber composition. As the electrically conductive material, examples are a carbon material such as carbon black, graphite powder or carbon fiber, and a metal powder.
However if the carbon black is admixed in an amount sufficient for obtaining electric conductivity, there occurs a problem that hardness of the rubber is increased because of a structure of carbon black. The mixing of powder such as graphite powder and carbon fiber having anisotropic shape causes a problem that roughness of the rubber surface is increased, and the admixing of metal powder causes a problem that an excellent chemical resistance inherent to the fluorine-containing rubber is impaired.
Also the desired electric conductivity varies depending on uses. In the above-mentioned various uses, the lower the volume specific resistance is, the more preferable if it is not more than 10.sup.8 .OMEGA.cm. The composition is selected within such a range as not sacrificing other properties of the fluorine-containing rubber composition.
For example, in a fixing roller of an electrostatic copying machine, the lower the volume specific resistance is, the more preferable if it is not more than 10.sup.8 .OMEGA.cm as mentioned above in order to prevent electostatic offset (a phenomenon wherein the roller is electrically charged due to a friction to paper, and an un-fixed toner image is sucked or repels due to this static electricity, thus disturbing the fixed image). At the same time, an excellent non-tackifying property is required to prevent hot offset (a phenomenon wherein the fixed image is disturbed because the toner heated and molten by the roller sticks to the roller).
For this purpose, attempts to add the carbon materials have been hitherto made, but the addition of the carbon materials in an amount enabling sufficient conductivity to be obtained causes a problem that non-tackifying property lowers. Thus a composition satisfying both electric conductivity and non-tackifying property is desired.
However a specific range of electric conductivity is desired in so-called semi-electrically conductive rollers of the electrostatic copying machine such as a charging roller, transferring roller and developing roller. Concretely in the charging roller, the volume specific resistance is required to be controlled to be in the range of 10.sup.8 to 10.sup.12 .OMEGA.cm. It is not allowed that even a part of the roller deviates from this range. That is to say, if it is not less than 10.sup.12 .OMEGA.cm, a necessary electric charge is not given to a photosensitive drum, and if it is not more than 10.sup.8 .OMEGA.cm, the photosensitive drum and high voltage power source are damaged by an over-current.
In order to use a fluorine-containing composition wherein usual electrically conductive substances are mixed, for these applications, there is a problem that accurate control of the mixing amounts and sufficient kneading are required. This is because it is difficult to control the volume specific resistance to be in a narrow range of 10.sup.8 to 10.sup.12 .OMEGA.cm since the resistance lowers sharply when the usual electrically conductive substance added step by step exceeds a certain amount. Therefore there is desired the fluorine-containing composition having electric conductivity and non-tackifying property which enables changes in mechanical properties such as increase in hardness and a surface roughness and non-tackifying property to be minimum without impairing inherent excellent properties of the fluorine-containing rubbers such as chemical resistance.
On the contrary, fluorine-containing rubber paints are widely used as industrial materials to be coated or impregnated on various substrates, for example, metals, plastics, rubbers, glasses, ceramics, fabrics, non-woven fabrics, fibers, porcelains and others because of excellent heat resistance, weatherability, oil resistance, solvent resistance and chemical resistance which fluorine-containing rubbers have.
Usual fluorine-containing rubber paints are those prepared by mixing an aminosilane compound as a coupling agent in an aqueous dispersion of fluorine-containing rubber as mentioned in JP-B-53671/1983, and if necessary, various additives are added thereto.
For instance, in JP-B-35432/1987, it was found out that by mixing a fluorine-containing resin, non-tackifying property and lubricity could be given to a surface of a coating without impairing adhesivity to a substrate, and further in addition, the coating having an excellent compression recovering property.
Also in JP-B-38393/1987, it was found out that electric conductivity could be imparted on an obtained film without impairing any features of fluorine-containing rubber paints by mixing an electrically conductive substance selected from the group consisting of a carbon black, a graphite, a metal and an antistatic agent.
However as mentioned hereinabove, it is required that fluorine-containing rubber paints have a particular range of volume specific resistance as the paints for the production of semi-electrically conductive rollers for electrostatic copying machine. Therefore when it is intended to control electric conductivity by mixing carbon materials such as carbon black as the electrically conductive substance in the fluorine-containing rubber paints, there is a problem that accurate control of the mixing amount and homogeneous dispersion are required. That is to say, this is because when usual electrically conductive substances added step by step exceed a certain amount, the resistance changes sharply. Among various electrically conductive substances, the carbon black has a feature that the deterioration of non-tackifying property, chemical resistance and the like of fluorine-containing rubbers can be minimized. However the carbon black being excellent in electric conductivity has a progressed structure, and is in difficult to disperse fluorine-containing rubber paints and larger in increase in viscosity. Also there is a marked tendency that the fluorine-containing rubber becomes harder and the surface rougheness is increased.
When the hardness increases, in order to obtain an effective contact to the drum and paper, it is necessary to press by a strong force, and as a result, there occurs disturbance of the drum and image. When the surface roughness is increased, there occurs irregular contact to the drum and paper and disturbance of the image.
Therefore there is required fluorine-containing rubber paints having electric conductivity and non-tackifying property which can minimize changes in the surface roughness, non-tackifying property and mechanical property such as hardness without causing a sharp change in the resistance by a change in the mixing amount and impairing excellent properties inherent to fluorine-containing rubber such as chemical resistance.
As mentioned above, an electrically resistive layer having electric conductivity and non-tackifying property is strongly demanded for semi-electrically conductive rollers for electrostatic copying machine. This matter is stated further concretely.
A corona discharge method and contact charge method have been adopted in a charging or transferring process of an electrostatic copying machine, laser printer and facsimile which utilize an electro-photographic method as one of image forming methods. First, in the corona discharge method, a machine structure is simple, but there are disadvantages that there is a generation of ozone, which not only causes an adverse effect on environment but also shorten a life of an organic photoreceptor. On the contrary, in the contact charge method, because there is no ozone generation and in addition, energy loss is small, a high pressure power source can be small-sized. In addition, since power consumption is decreased, this method is suitable for making a size of a whole machine smaller. The contact charge method in the charging process is one for electrically charging a surface of a photosensitive drum by bringing the semi-electrically conductive roller into contact with a surface of a photosensitive drum and rotating it. This method is proposed in, for instance, JP-A-843/1975, JU-A-88645/1983, JP-A-194061/1983, JP-A-142569/1989 and JP-A-311972/1992.
An electrostatic copying machine using this contact charge method has a structure as stated, for instace, in JP-A-311972/1992. The structure thereof is, for example, one as shown in FIG. 1. A photosensitive drum 1 usually comprises an organic photoreceptor, and there may be used selenium, CdS, amorphous silicon and the like. A charging roller 2 is disposed being brought into contact with the above-mentioned photosensitive drum 1. A developing device 3, a transferring roller 5 and a toner cleaner 7 are arranged in the clockwise direction, certering around the charging roller 2. Further a fixing roller 6 for fixing of a transferred paper 4 fed out from the transferring roller 5 is disposed in the vicinity of the photosensitive drum 1 between the transferring roller 5 and the toner cleaner 7.
The image forming process of the electrostatic method using the semi-electrically conductive roller is briefly explained hereinbelow.
The charging roller 2 the comprising the semi-electrically conductive roller having an elasticity is rotated on the outer circumferential surface of the photoconductive drum 1 (for instance, linear speed 60 mm/sec) rotating in the direction of an arrow, by the photosensitive drum 1, being partly elastically deformed. The outer surface of the photosensitive drum 1 is electrically charged by bringing it into contact with this charging roller 2. On the surface of the thus charged photosensitive drum 1 is formed an electrostatic latent image corresponding to an original image by means of an exposing mechanism portion 8, and the latent image is made into a visible image by a developing device 3. Then an electric charge reverse to the visible image of toner particles which is formed on the photosensitive drum 1 is applied to a transfer paper through the transferring roller 5 to transfer the visible image of the toner particles onto the transfer paper 4. The visible image of the toner particles electrostatically sticking to the transfer paper 4 is fused and deposited on the transfer paper 4 by the heated fixing roller 6 to give a fixed image.
In this case, 85 to 95% of the toner sticking onto the surface of the above-mentioned photosensitive drum 1 by means of the transferring roller 5 is transferred on the drum but the remaining toner after this transfer is nearly completely removed by means of a toner cleaner 7, then is wholly subjected to an emission of light by an eraser 9, and initiated to make preparation for the subsequent charging.
As mentioned above, for the electrostatic copying machine and the like, there are used many semi-electrically conductive rollers such as the charging roller, developing roller, transferring roller and fixing roller. As shown in FIG. 2, such a semi-electrically conductive roller to be used is such that a metallic core roll and an electrically conductive elastic layer 11 on the outer circumference thereof are formed and an electrically resistive layer 12 is formed over this electrically conductive elastic layer 11.
As the materials for the electrically conductive elastic layer, there are used ones having a volume specific resistance of not more than 10.sup.5 .OMEGA.cm, preferably not more than 10.sup.3 .OMEGA.cm and a rubber hardness (JIS A) of 20 to 50 degrees, preferably 25 to 40 degrees. This layer comprises generally a composition prepared by mixing an electrically conductive powder (carbon black, metal powder and the like) into a synthetic rubber such as a silicone rubber, ethylene propylene rubber, nitrile rubber and urethane rubber.
Also as the materials for the electrically resistive layer, those having a volume specific resistance of 10.sup.6 to 10.sup.12 .OMEGA.cm, preferably 10.sup.8 to 10.sup.12 .OMEGA.cm are used. This resistive layer generally comprises a polar rubber such as an epichlorohydrin rubber, nitrile rubber, acrylic rubber, urethane rubber and chloroprene rubber, or further a composition prepared by mixing a low resistive substance such as an electrically conductive powder (carbon black, metal powder and the like ), electrically conductive fiber (carbon fiber and the like), fluorine-containing surfactant and ester type plasticizer into a highly resistive synthetic rubber such as a silicone rubber, ethylene propylene rubber, styrenebutadiene rubber, or a fluorine-containing polymer such as a fluorine-containing resin and fluorine-containing rubber.
However in the semi-electrically conductive roller having the electrically resistive layer as mentioned above, when the resistive layer comprises a polar rubber such as a epichlorohydrine rubber, nitrile rubber and acrylic rubber, since a releasing property against a toner is generally poor, there is a problem that a small amount of the toner remaining on the surface of the photoreceptor sticks to the roller surface and is solidified (This phenomenon is called a toner filming.) when this roller is used as a charging roller and a transferring roller. For instance, in case where the toner is solidified on the charging roller, a roller charger is deprived of its function, and it becomes impossible for the portion of the roller, where the toner sticks and is solidified, to electrically charge the photoreceptor. Also when used as the developing roller, there occurs more remarkably the sticking of the toner to the roller surface, which becomes a cause for an irregular image and is not preferable.
Also in case where the materials of the electrically resistive layer are ones prepared by dispersing a carbon black or the like in a synthetic rubber or a fluorine-containing polymer, there is a disadvantage that an electric breakdown is liable to occur under high voltage. For instance, in case of the charging roller, if there are pin holes on the surface of the photoreceptor, there is formed an electrically conductive pass leading to a back plate of the photoreceptor, thus excess current flows from the charging roller and a voltage applied to the charging roller drops. From a point of an image, in a negative-positive phenomenon, there is a problem that a black line appears in the horizontal direction of a contacting portion between the photoreceptor and the charging roller.
This occurs due to the reason mentioned hereinbelow. As mentioned above, when adding the carbon black to the insulative substrate, the volume specific resistance of the obtained composite material sharply changes from not less than 10.sup.12 .OMEGA.cm down to not more than 10.sup.6 .OMEGA.cm when the carbon black to be added exceeds a certain amount. Therefore if the dispersion of carbon black is insufficient, there occurs locally a difference in an adding amount and there is a portion where the volume specific resistance is not more than 10.sup.6 .OMEGA.cm. In this portion there occurs an electric breakdown arising from an insufficient dispersion of the carbon black.
Also when a fluorine-containing surfactant and an ester type plasticizer are added to the materials of the electrically resistive layer, these additives bleed out with the lapse of time. Thus not only an initial characteristic of the roller cannot be maintained for a long period of time, but also the surface of the photoreceptor is stained and a life of a whole system is also shortened.
An object of the present invention is to provide a composition and paint composition being capable of imparting electric conductivity and non-tackifying property to a resin or a rubber.
Also an object of the present invention is to provide a semi-electrically conductive roller being excellent in releasing property (non-tackifying property) against a toner and having an electrically resistive layer, which comprises materials containing neither an electrically conductive powder such as carbon black being liable to cause an electric breakdown under high voltage nor a low resistive substance such as a fluorine-containing surfactant and ester type plasticizer being liable to ooze out from the inside of the roller with the lapse of time and stain the photoreceptor and the like.